Well perforating with abrasive fluids



March 30, 1965 w. HOUGH ETAL WELL PERFORATING WITH ABRASIVE FLUIDS 4Sheets$heet 1 Filed Aug. 26, 1960 sum "23 WATER-27 rum a auzuozn 20-m'rnocsn FIG.

PRIOR ART INVENTORS w. HOUGH THOMPSON, RHEA, PARKER,

FIG 3 ELDR GENE JOHN JACK BY fb ATTORNEY.

United States Patent Ofilice 3,175,613 Patented Mar. 30, 1965 3,175,613WELL PERFORATING WITH ABRASIVE FLUID Eldred W. Hough, Austin, and GeneD. Thompson, John W. Rhea, and Jack D. Parker, Houston, Tex., assignors,

by mesne assignments, to Jersey Production Research Company, Tulsa,Okla, a corporation of Delaware Filed Aug. 26, 1960, Ser. No. 52,150 7Claims. (Cl. 166-35) This invention relates generally to perforatingtechniques for opening fluid communication between a well conduit and anearth formation penetrated by the well, and more particularly to the useof abrasive hydraulic jets for such perforating techniques.

After a borehole has been drilled in the earth for the purpose ofrecovering oil, gas, and other mineral deposits therein, it is customaryto cement a pipe string in the borehole to serve as a liner for theborehole. In order to produce the formation, it then becomes necessaryto perforate the pipe, the cement sheath, and the earth formation toopen passages for the flow of fluid from the earth formation into thewell bore.

Various methods and apparatus have been used in the past for the purposeof perforating wells. One technique that has been found to be successfulutilizes an abrasive liquid which is forced downwardly into the wellthrough a tubing string, and which is directed laterally from the tubingstring against the well casing by means of specially designed nozzles.The abrasive liquid stream is usually water or oil charged with sand inthe ratio of between A; to 3 or 4 pounds per gallon of liquid. Theliquid stream is ejected from the nozzles for any desired length of timeto abrade the wall of the well casing, the cement sheath therearound,and the productive earth formation from which it is desired to produceformation fluids. The general technique is described in US. Patents No.2,758,- 653, Desbrow, No. 2,302,567, ONiel, and No. 2,315,496, Boynton.

The abrasive fluid-jet technique of perforating appears to functionquite well under conditions where there is no fluid or hydrostatic backpressure against which the jet must operate and as long as it ispossible to have free flow of fluids out of and away from the holedrilled by the abrasive fluid. This is evidenced by the results reportedin an article in the Oil and Gas Journal, June 15, 1959, at page 68.However, under well conditions, a very different situation prevails. Ithas been conclusively determined that the penetration obtained with anabrasive liquid is substantially reduced when the back pressure againstwhich the jet is directed exceeds 20 p.s.i.

In accordance with the teachings of the present invention, the abrasiveliquid stream is mixed with a normally gasiform fluid prior to beinginjected into the tubing string carrying the abrasive liquid down to theearth formation to be perforated. By normally gasiform fluids is meant afluid which is a gas at normal (or atmospheric) conditions oftemperature and pressure at the earths surface. Fluids in this categoryinclude hydrogen, nitrogen, carbon dioxide, hydrocarbon gases and thelike. The volume of the normally gasiform fluid introduced into theabrasive fluid stream is such that the fluid stream is at least 25percent saturated with the normally gasiform fluid at the temperatureand pressure conditions just upstream from the nozzle through whichfluid is directed against the earth formation. By "25 percent saturatedis meant that the liquid, at a given temperature and pressure, contains25 percent of the maximum volume of gas (measured at standardtemperature and pressure) that will go into solution in the liquid atthe temperature and pressure of the liquid. The fluid stream iscompletely saturated with the abrasive fluid for maximum rate ofpenetration and total penetration.

However, a 25 percent solubility ratio has been found to be effective.The liquid may be any liquid adapted to dissolve the normally gasiformfluid to be used. Water, either fresh or saline, and diesel oil will befound to be quite effective in this regard. The abrasive agent mixedwith the fluid may be sand, emery grains or other hard, abrasivematerials.

The invention will be further described with reference to theaccompanying drawings wherein:

FIG. 1 is a diagrammatic representation of a well installation forpracticing the technique of the present invention;

FIG. 2 is a cross-sectional representation of a perforation being cut byabrasive liquid utilizing prior art techniques;

FIG. 3 is a cross-sectional representation of a perforation being cututilizing the technique of the present invention; and

FIGS. 4 through 9 are graphs of perforating time in minutes as afunction of depth of perforation in inches for a variety of types ofcement and earth formation cores under various conditions of backpressure, gasiform fluid injection, and pressure drop across aperforating nozzle.

In FIG. 1 there is shown a borehole 1 drilled from the surface of theearth to a productive earth formation 35. A casing pipe string 3 isshown as having been cemented to the sides of the borehole. A tubingstring 5 coextends with the casing pipe string 3 from the earths surfaceand is supported at the earths surface by conventional wellheadapparatus 15. At the lower end of the pipe string there is connected aheavy steel tube 31 similar to a small casing shoe or drill collar.Ports in the heavy steel tube 31 are fitted with hard steel plugs 33which act as nozzles for directing fluids from the lower end of thetubing and the heavy steel tube 31 laterally at the sides of the casingand the earth formation 35. The tubing string 5 is in fluidcommunication through the wellhead 15 and a pipe 13 with the output of ahigh pressure pump and liquid-sand blender 21; The inlet of the pump andblender 31 is connected to a liquid reservoir 27 by a pipe 29, and to anabrasive material (such as sand) reservoir 23 by a pipe 25. Exhaust pipe7 is connected through the wellhead 15 to the annulus between the tubingstring 5 and easing string 3 so that fluids may be exhausted from theannulus into a mud pit or other reservoir. A pressure gauge 9 isconnected to the annulus by means of a pipe 11 and valve 12. Theapparatus described above is conventional and may be of the typenormally used by the Dowell Company for the so-called Abrasijet systemof abrasive fluid perforation.

The pump 21 should be capable of pumping abrasive fluid down the tubingstring 5 under 2000 to 5000 pounds per square inch of hydraulicpressure. The blender mechanism included with the pump 21 should beadapted to mix sand and water (or other liquid) in the ratio of between0.5 to 3 pounds of sand to each gallon of liquid drawn from reservoir27.

A source 20 of high pressure, normally gasiform fluid, is connected tothe outlet of pump 21 (or to pipe 13) by means of a pipe 17 and valve19. The normally gasiform fluid may be hydrogen, nitrogen, carbondioxide, natural gases such as methane, or other gasiform fluid that iscapable of being dissolved in the liquid stored in reservor 27. Whennitrogen is used as the normally gasiform fluid, it may be under apressure of 6000 to 8000 p.s.i.

When it is infeasible or undesirable to use highly pressurized gases, itis feasible to use a chemical adapted to react quickly with the liquidfrom reservoir 27 to form the gas in the desired solution ratio. Forexample, a metal hydride, preferably in powdered form, may be injectedinto the liquid stream to react with water to liberate hydro- "gen.Examples of complex metal hydrides that may be used for this purpose arelithium aluminum hydride, lithium boron hydride sodium boron hydride,and sodium aluminum hydride." Examples of primary hydrides that may beused arelithiurn hydride, sodium hydride, calcium hydride and potassiumhydride. The chemical may be mixed with the abrasive agent in reservoir23 or may be injected into pipe 13 from a separate container.

When the apparatus is to be operated, the pump is started and the "valve19 is open so that the fluid pumped down tubing string is at least 25%saturated with the normally gasifor'm fluid. As indicated above, theabrasive liquid stream is preferablycompletely saturated with thenormally gasiform fluid from source 20. The nozzles 33 direct the fluidlaterally against the casing 3 and, in due course, holes will be boredthrough the casing by the erosive or abrasive action of the fluid,through the cement around the casing, and into the earth formation 35.The fluid pumped down the tubing string 5 is maintained at a pressuresuch that the pressure in the fluid stream at the mouth of each nozzleis less than the bubble-point pressure of the gasiform fluid in theliquid stream in the tubing string. After a suitable interval of time,usually 20 to 40 minutes, the pump may be shut off and the valve 19closed. If it is desired, the tubing string 5 may be left in the welland used as'a production conduit if suitable production packer means(not shown) is connected to tubing string 5 above the heavy steel tube31.

It has been found that the perforations made in accordance with thepresent invention are drilled much faster and to a much greater depththan are the perforations formed in accordance with prior arttechniques. For an explana doubt this statement, reference is made toFIGS. 2 and 3. In FIG. 2 there is shown a perforation being formed inaccordance with prior art techniques. The nozzle 41 is shown as havingdirected an abrasive liquid stream against a casing 43, a cement section45 and an earth for- 'mation 47. In FIG. 3 the abrasive fluid 39 isformed in accordance with the present invention and may consist of thesame relative quantities of sand and water into which has been dissolveda normally gasiform fluid, such as nitrogen.

It will be noted that the perforations formed in the sandstone are ofdifferent contour. The perforations are assumed to have been drilled forsubstantially the same length of time. The opening in the sandstoneformed in accordance with prior art technique is wider and not nearly asdeep as that formed in accordance with the present invention. It ispostulated that the action of the prior art fluid jet has been to form afluid hammer ahead of the jet which is driven against the earthformation without particular effective scouring action by the sand ladenfluid 37. In other words, a bank of fluid which is more or less of thesame liquid is initially formed which simply is rammed into the earthformationby the fluid jet. The bank of liquid is held in place by thehydroa static back pressure of the liquid in the well.

Consider now the action of the fluid jet shown in FIG. 3. As soon as thefluid stream begins to leave the nozzle 41, the normally gasiform fluidin the liquid comes out of solution. Shortly after the gasiform fluidhas exited from the nozzle 41, large bubbles 40 are formed in the fluidstream. As the bubbles are directed down the fluid jet at the formation,they gradually reduce in size. However, many bubbles remain in the fluidstream to impinge against earth formation. The reason for thisphenomenon is believed to be related to the pressure gradient in thejet. Before the fluid passes through the nozzle, the pressure is veryhigh. At the mouth of the nozzle the pressure is at a minimum. As thevelocity of the fluid decreases after passing through the nozzle, thepressure within the fluid increases. Bubbles which are formed at themouth of the jet by the gas coming out of solution are compressed bythis pressure increase. When the compressed bubbles reach the end of thefluid stream their velocity is'reduced to zero, and the energy thereinis released into the liquid or earth that stops their forward travel.The effect of these bubbles is to create a series of shock waves whichinduce a spalling attack on the rock present at the liquid boundary atthe earth formation that is effective to break up any fluid bank thatattempts to form at the surface of the formation in the perforation.Thus, the bubbles and sand particles are able to strike the earthformation rather than to be deflected away' from the formation by thefluid bank, as happens when gasiform fluid is not mixed with the abrasive fluid stream. The cross-section of the perforation formed in theearth' formation is narrower and far more pointed than the perforationformed by the use of prior art techniques.

From the above discussion it can be' seen that the erosion thus is acombination of the erosion produced by the sand particles and erosion bycavitation.

The curves of FIGS. 4 through 9 summarize the results obtained bydirecting fluid jets at neat cement cores and various types of earthformation cores, The same standoff distance was used to obtain the datafor all of the curves. Various back pressures were produced at the faceof the core being tested to simulate the hydrostatic pressure of theborehole fluids as would be found under actual operating conditions inthe field. The curves of FIG. 6 were obtained from substantiallyidentical cores of neat cement that had been allowed to age for fivedays. The curves of FIGS. 4 and 8 were obtained utilizing Bereasandstone cores. The curves of FIGS, 5, 7, and 9 were obtained utilizingcores of Austin chalk, Indian limestone and Carthage limestone,respectively.

A number of conclusions can be reached from the curves of FIGS. 4through 9. It is readily apparent that both total penetration and therate of penetration are substantially increased by the use of gasiformfluid injected into the abrasive fluid mixture. This is particularlyapparent from the results shown in FIGS. 5, 8, and 9. In the experimentssummarized by the curves of each of these figures, a very small backpressure of 300 or 400 pounds per square inch was enough to decrease thetotal penetration and rate of penetration to between /3 and /2 of thetotal penetration and rate of penetration, respectively, obtained withzero back pressure.

Furthermore, the use of 400 cubic feet per minute of nitrogen injectedinto the fluid stream with 300 or 400 pounds of back pressure at thecore was enough to increase the total penetration and the rate ofpenetration to greater than what they were without nitrogen injectionand with zero back pressure. While the increase of back pressure tobetween 1000 and 2000 p.s.i. substantially reduced both totalpenetration and rate of penetration, the reduction did not reach thatobtained without nitrogen injection and with 200 or 300 pounds of backpressure at the core.

Furthermore, from a consideration of the curves of FIG. 4, it can beseen that with the back pressure of 300 or 400 p.s.i. and with variousamounts of injected nitrogen, that even 25% or 50% nitrogen saturationwas suflicient to substantially increase the total penetration and therate of penetration from that obtained without nitrogen injection. Forthe conditions set forth in FIG. 4, saturation was reached at about 375or 400 cubic feet per minute of injected nitrogen. It is also apparentfrom the results of FIG. 4 that not very much improvement in totalpenetration and rate of penetration couldbe expected by increasing theamount of injected nitrogen] past saturation. This can be seen bycomparing curves C and D where the results of curve C were obtainedusing substantially saturated fluid, and the results of curve D wereobtained utilizing substantial amountsof nitrogen substantially inexcess of that required to saturate thefluid.

It is to be noted that on all curves only those labeled as havinginjected nitrogen were obtained with fluids having nitrogen injectedtherein. Curves A, E, F, J, K, N, 0, Q, R, U and V were obtained with nonitrogen being injected in the abrasive fluid.

It can be seen from the results summarized in FIG. 6 that with neatcement substantially the same penetration was realized when the backpressure was increased from zero p.s.i. to 225 p.s.i. without nitrogeninjection. The use of nitrogen injection, however, dramatically andunexpectedly increased both total penetration and rate of penetration.Increasing the back pressure to 2400 psi. substantially reduced thepenetration and penetration rate but the reduction, as shown by curve M,was insufiicient to reduce total penetration and rate of penetration tothat obtained with zero back pressure and no nitrogen injection.

The invention is not necessarily to be restricted to the specificstructural details or arrangement of parts, as various modificationsthereof may be effected without departing from the spirit and scope ofthe invention.

The objects of the invention having been described above, what it isdesired to claim is:

1. An improved method of fluid abrasive-jet perforating the casing of acased, liquid-containing borehole and a hydrocarbon productive earthformation penetrated by the borehole comprising: positioning at leastone fluid nozzle in the borehole at the level of the hydrocarbonproductive earth formation; continuously forcing down the pipe stringand up the annulus around the pipe string a pressurized, abrasive liquidstream containing a pressurized gasiform fluid soluble in the liquid insaid liquid stream to direct at the formation through said at least onenozzle said pressurized, abrasive liquid stream containing a pressurizedgasiform fluid; and maintaining the pressure of the liquid stream beforeit is directed through said nozzle at greater than the pressure requiredto maintain the pressure in the liquid stream exiting from the nozzle atless than the bubble point pressure of the gasiform fluid in the liquidstream.

2. In a method of perforating a cased, liquidcontaining borehole and asurrounding earth formation with a well installation including a pipestring in the borehole, equipped with at least one nozzle at the depthof the formation for directing the pressurized abrasive liquid streamlaterally of the pipe string at the borehole casing and formation, theimprovement comprising: continuously injecting the abrasive liquidstream into the pipe string under high pressure and flowing the abrasiveliquid stream up the annulus around the pipe string to the earthssurface; injecting a normally gasiform fluid into the abrasive liquidstream before it is injected into the pipe string at a volume ratesufficient to bring the gasiform fluid-liquid ratio to at least 25% ofgasiform fluid saturation in the liquid, and maintaining the pressure ofthe liquid stream being injected in the pipe string at greater than thepressure required to maintain the pressure in the liquid stream at eachopening of said at least one nozzle at less than the bubble pointpressure of the gasiform fluid in the liquid stream in the pipe stringwhereby the gasiform fluid comes out of solution as it exits from saidat least one nozzle.

3. The method of claim 2 wherein the gasiform fluid is nitrogen.

4. The method of claim 2 wherein the gasiform fluid is carbon dioxide.

5. The method of claim 2 wherein the gasiform fluid is an aliphatichydrocarbon and the liquid is a nonhydrocarbonaceous liquid.

6. The method of claim 2 wherein the gasiform fluid is hydrogen.

7. The method of claim 6 wherein the liquid stream is water and thegasiform fluid is generated by injecting a metal hydride into the liquidstream.

References Cited by the Examiner UNITED STATES PATENTS 2,302,567 11/42ONeill 166-35 2,315,496 4/43 Boynton l66--35 2,758,653 8/56 Desbrow.

2,796,129 6/57 Brandon 166-9 2,876,839 3/59 Fast et a1. l6622 2,889,8846/59 Henderson et al l6638 CHARLES E. OCONNELL, Primary Examiner.

BENJAMIN BENDETT, Examiner.

1. AN IMPROVED METHOD OF FLUID ABRASIVE-JET PERFORATING THE CASING OF ACASED, LIQUID-CONTAINING BOREHOLE AND A HYDROCARBON PRODUCTIVE EARTHFORMATION PENETRATED BY THE BOREHOLE COMPRISING: POSITIONING AT LEASTONE FLUID NOZZLE IN THE BOREHOLE AT THE LEVEL OF THE HYDROCARBONPRODUCTIVE EARTH FORMATION; CONTINUOUSLY FORCING DOWN THE PIPE STING ANDUP THE ANNULUS AROUND THE PIPE STRING A PRESSURIZED, ABRASIVE LIQUIDSTREAM CONTAINING A PRESSURIZED GASIFORM FLUID SOLUBLE IN THE LIQUID INSAID LIQUID STREAM TO DIREC AT THE FORMATION THROUGH SAID AT LEAST ONENOZZLE SID PRESSURIZED, ABRASIVE LIQUID STREAM CONTAINING A PRESSURIZEDGASIFORM FLUID; AND MAINTAINING THE PRESSURE OF THE LIQUID STREAM BEFOREIT IS DIRECTED THROUGH SAID NOZZLE AT GREATER THAN THE PRESSURE REQUIREDTO MAINTAIN THE PRESSURE IN THE LIQUID STREAM EXITING FROM THE NOZZLE ATLESS THAN THE BUBBLE POINT PRESSURE OF THE GASIFORM FLUID IN THE LIQUIDSTREAM.